1. Field of the Invention
This invention relates to an encapsulated elecrtrostatographic toner material, and more particularly to an encapsulated electrostatographic toner material advantageously employable in a pressure fixing process.
2. Description of Prior Arts
There is known an electrostatography which comprises a stage of developing a tone electrostatic latent image contained on a photoconductive or dielectric surface with a toner material containing a colorant and a fixing aid (i.e., binder) to produce a visible toner image, and a subsequent stage of transferring and fixing the visible toner image onto a surface of a support medium such as a paper sheet.
The development of the latent image to produce a visible toner image is carried out by the use of either a developing agent consisting of a combination of a toner material with carrier particles, or a developing agent consisting of a toner material only. The developing process utilizing the combination of a toner material with carrier particles is named "two component developing process", while the developing process utilizing only a toner material is named "one component developing process".
The toner image formed on the latent image is then transferred onto a surface of a support medium and fixed thereto. The process for fixing the toner image to the support medium can be done through one of three fixing processes, that is, a heat fixing process (fusion process), a solvent fixing process and a pressure fixing process.
The pressure fixing process which involves fixing the toner material onto the surface of a support medium under application of pressure thereto is described, for instance, in U.S. Pat. No. 3,269,626. The pressure fixing process involving the use of neither a heating procedure nor a solvent produces no such troubles as inherently attached to either the heat fixing process or the solvent fixing process. Moreover, the pressure fixing process can be employed with a high speed automatic copying and duplicating process, and the access time is very short in the pressure fixing process. Accordingly, the pressure fixing process is considered to be an advantageous fixing process inherently having a variety of preferable features.
However, the pressure fixing process also has certain inadvantageous features. For instance, the pressure fixing process generally shows poorer fixablity than the heat fixing process does, whereby the toner image fixed onto a paper is apt to rub off easily. Further, the pressure fixing process requires very high pressure for the fixing operation, and such high pressure tends to break the cellulose fibers of the support medium such as paper sheet and also produces glossy surface on the support medium. Moreover, the pressing roller requires to have relatively greater size, because the roller necessarily imparts very high pressure to the toner image placed on the support medium. Accordingly, reduction of the size of a copying and duplicating machine cannot exceed a certain limit defined by the size of a pressing roller.
There has been previously proposed an encapsulated toner material which comprises toner particles enclosed with microcapsules, so as to overcome the above-described disadvantageous features of the pressure fixing process. The encapsulated toner material is generally prepared by enclosing a core material (containing a colorant such as carbon black) with a shell which is rupturable by the application of pressure in the developing stage. Thus prepared encapsulated toner material has various advantageous features; for instance, fixing of the encapsulated toner material does not require very high pressure but the fixability is high. Accordingly, the encapsulated toner material is viewed as suitable for the use in the pressure fixing process. However, the encapsulated toner materials proposed up to now appear unsatisfactory in practical use, because they fail to meet some of requirements required for providing smooth copying and duplicating operation and satisfactory toner image fixability and quality.
More in detail, it is required for the toner material for the use as a dry type developing agent in the electrostatography to have excellent powder characteristics (or, powder flowability) to provide high development quality, and to be free from staining the surface of a photosensitive material on which a latent image is to be formed.
Further, a toner material employed for the two component developing process is also required not to attain the surfaces of the carrier particles employed in combination. The toner material for the use as a developing agent in the pressure fixing process is furthermore required to be satisfactory in the fixability under pressure and not to undergo off-setting on the roller surface, that is, phenomenon that the toner adheres to the roller surface so as to stain it.
In summary, a toner material employed in the pressure fixing process ought to be at a high level in all characteristics such as powder characteristics (i.e., powder flowability), fixability onto a support medium (e.g., paper sheet) as well as presevability of the fixed image, resistance to the off-setting, and electron chargeability and/or electroconductivity depending on the system employed. The previously proposed encapsulated toner materials are unsatisfactory in some of these characteristics.
Moreover, an encapsulated toner to be employed in the one component developing system should contain a particulate magnetizable substance such as ferrite or magnetite in such an amount of approx. 40-60 wt. % in the core. However, it is rather difficult to enclose such a large amount of the particulate magnetizable substance within a microcapsule. In more detail, an encapsulated toner is generally prepared by dispersing (or emulsifying) an oily liquid containing core materials in an aqueous medium and subsequently forming a shell around the produced oily droplet of the core material. In this process, since the particulate magnetizable substance has hydrophilic property, it is difficult to completely enclose such large amount of the hydrophilic particulate magnetizable substance within the shell without releasing a portion of the substance outside.
With respect to the above-described problem, there has been proposed a process which comprises treating the surface of the particulate magnetizable substance to become hydrophobic and forming a shell around an oily core material containing the so hydrophobically treated particulate magnetizable substance by a surface polymerization process in U.S. Pat. No. 4,307,169. However, it is noted that the encapsulated toner prepared by the process is liable to show deterioration of the fixability and powder characteristics (or powder flowabity) in a storage for severe conditions such as 16 hours at 100.degree. C. or a long period such as one month at room temperature, because the oily material contained in the core evaporates through the shell.